The Global Positioning System (GPS) is undergoing a transformation in terms of satellite hardware and code signals used. This transformation includes a redefinition of GPS from a military service with the guarantee of civil use to a true dual service. The transformation includes a new civil L5 frequency and a military acquisition signal at the L1 and L2 frequencies for enhanced military and civil use.
The new M-code signal at the L1 and L2 frequencies consists of the product of a military data modulation multiplied by a spreading code modulation. The M-code signal is designed to be more robust and be more usable than the present P-code, Y-code system in current use. The M-code waveform places signal energy in the nulls of the current Y-code signal, for example, see FIG. 1 which depicts the present P-code and Y-code signals and the C/A code signals. FIG. 2 depicts the M-code signals which, as stated previously, fills the nulls of the P-code and Y-code signals. Like the current Y-code signal, M-code is only for authorized users and is not available to the general public outside of the military. The M-code signal modulation is designed to provide enhanced capability for military users of GPS. Currently, the GPS L1 frequency (1575.42 MHz) has two signals modulated on it, the C/A-code and the P/Y-code. The spectra of these modulations are characterized by a strong central peak surrounded by deep nulls at the modulation chip rate and its multiples. For C/A code, the chip rate is 1.023 MHz and for P/Y-code, the chip rate is 10.23 MHz. The GPS L2 frequency (1227.6 MHz) is similar but currently has only P/Y-code modulation. In the future, L2 may include a C/A-code signal to allow civil use of the frequency. M-code is a split spectrum signal with little energy at the carrier frequency and major lobes spaced away from the carrier. The side peaks in the modulation spectrum are intentionally put in nulls of the P/Y-code. This has the benefit of minimizing interference between the new signal and the old ones. It also is specifically designed to keep military signal energy away from the civil C/A-code signal. The M-code signal will be able to operate at powers tens of decibels higher than the current signals.
As always, in any signal processing system, it would be beneficial to provide for improved signal-to-noise ratio for the GPS receiver. Thus, using the new M-code, which provides the availability of side lobes in the autocorrelation function, as compared to the single peak of the conventional P/Y-code correlation function, improvements may be made. There is a need for a GPS detector which uses the information in the side lobes to improve signal-to-noise ratio in the output function. Further, there is a need for providing extra taps on the autocorrelation function side lobes to provide for better performance of the GPS receiver.
It would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.